A thermal-type infrared solid-state image sensing device has a plurality of light receiving elements which are arranged in array. Each of the light receiving elements absorbs the infrared rays radiated from an object by use of a light receiving section (diaphragm) having microbridge structure, converts the infrared rays into heat, detects a temperature rise due to the heat by use of a thermoelectric conversion element, such as a bolometer, comprised in the light receiving section (diaphragm), and outputs the temperature rise as an electrical signal. The thermal-type infrared solid-state image sensing device outputs the outputs of the plurality of light receiving elements as time-series signals to the outside by use of a read-out circuit. The thermal-type infrared solid-state image sensing device measures the temperature distribution of the surface of the object by the distribution of the electrical signals thus obtained from the plurality of light receiving elements.
A thermoelectric conversion element, such as a bolometer, detects the intensity of infrared rays by a temperature change caused by the infrared rays which are absorbed, and hence if the environmental temperature and the like change, a drift occurs in the thermal-type infrared solid-state image sensing device, with the result that it becomes impossible to accurately detect the temperature of an object. In order to suppress the drift of an output of the thermal-type infrared solid-state image sensing device due to such variations in the environmental temperature and the like, it is necessary only that the temperature (environmental temperature) of the thermal-type infrared solid-state image sensing device be controlled by use of a temperature controller. However, in this method, it is necessary to separately provide a temperature controller and this complicates the structure of the device and makes the device expensive. Therefore, as a method of suppressing the drift of an output of the thermal-type infrared solid-state image sensing device without using a temperature controller, there have been contrived methods which involve providing a reference element which outputs a reference signal based on the environmental temperature and the like without being influenced by the effect of incident infrared rays and removing a drift caused by variations in the environmental temperature and the like by performing signal processing using the reference signal. The configuration of a reference element of a thermal-type infrared solid-state image sensing device capable of being used in the suppression of a drift caused by variations in the environmental temperature and the like (removal of drift components) without using a temperature controller is disclosed in JP 2009-192350 A.
Referring to FIGS. 6(a) and 6(b), a description will be given of an example of configuration of a conventional thermal-type infrared solid-state image sensing device disclosed in JP 2009-192350 A. FIG. 6(a) shows a sectional view of a light receiving element (first element 20a) which detects incident infrared rays, and FIG. 6(b) shows a sectional view of a reference element (second element 20b) which outputs a “reference signal” for correcting the “drift of an output” of the light receiving element. A circuit substrate 21 is formed from a Si wafer and the like, and a read-out circuit 21a is made in the interior thereof. An infrared reflective film 22 is formed on the circuit substrate 21 and a protective film (not shown) is formed in the upper layer thereof. The light receiving section (temperature detection section 33) is made up of a first protective film 25, a second protective film 27, and a third protective film 29, which absorb infrared rays having wavelengths in the vicinity of 8 μm to 12 μm, a bolometer thin film 26 surrounded by these protective films, and an electrode interconnect 28. A supporting section 32 is made up of the first protective film 25, the second protective film 27, the third protective film 29, and the electrode interconnect 28 which are surrounded by these protective films. In the light receiving element (first element 20a) shown in FIG. 6(a), the supporting section 32 supports the light receiving section (temperature detection section 33) in such a manner as to float the light receiving section (temperature detection section 33) in the air from the circuit substrate 21 via a hollow section 34, thereby realizing a heat separation structure.
In an initial step of device manufacture, the hollow section 34 of the light receiving element (first element 20a) is buried with a first sacrifice layer 24 which is patterned (in the example disclosed in JP 2009-192350 A, a first sacrifice layer made of DLC: diamond-like carbon). In a final step of device manufacture, the hollow section 34 is formed as a result of removal of the first sacrifice layer 24 (for example, the first sacrifice layer made of DLC) by dry etching. On the other hand, in the reference element (the second element 20b shown in FIG. 6(b), the patterned first sacrifice layer 24 remains as it is. As a result, a heat separation structure is not realized and in the reference element (second element 20b), the generated condition is such that the light receiving section (temperature detection section 33) and the circuit substrate 21 which is a heat sink are thermally connected.
Furthermore, in the light receiving element (first element 20a), a eave 31 which absorbs infrared rays having wavelengths in the vicinity of 8 μm to 12 μm extends outward from an end portion of the light receiving section (temperature detection section 33) and covers the upper part of the supporting section 32, thereby functioning as a “eave” which blocks the incidence of infrared rays. Also the infrared rays absorbed by this eave 31 are converted into heat, and the converted heat flows into the light receiving section (temperature detection section 33). Therefore, it is possible to effectively use the space of the supporting section 32 for the detection of infrared rays and it is possible to improve the aperture ratio. On the other hand, in the reference element (second element 20b), the eave 31 is caused to remain in the outermost layer, and this is a “eave” which covers the whole upper part of the temperature detection section 33 and supporting section 32, thereby blocking the incidence of infrared rays. By adopting the configuration in which this eave 31 remains in the outermost layer, it is possible to cause a second sacrifice layer 30 to remain. Furthermore, even when a first slit 35 and a second slit 36 are formed in the step of forming the light receiving section (temperature detection section 33) and the supporting section 32, the second sacrifice layer 30 and the outermost eave 31 remain, with the result that it is impossible to remove the first sacrifice layer 24 (for example, a first sacrifice layer made of DLC) by dry etching and hence it is possible to cause the first sacrifice layer 24 to remain.
In order to increase the accuracy of drift suppression (removal of drift components), it is desired that the resistance and the temperature coefficient of resistance of the bolometer thin film 26 of the light receiving element (first element 20a) and those of the reference element (second element 20b) be equal with each other under the same temperature condition. Because resistant materials used in the fabrication of the bolometer thin film 26 have a piezoresistance effect to no small extent, if the residual stresses of the materials constituting the light receiving section (temperature detection section 33) are not equal, a resistance difference and a difference in the temperature coefficient of resistance due to the piezoresistance effect occur, with the result that the accuracy of drift suppression (removal of drift components) decreases. Providing the first slit 35 and the second slit 36 also in the reference element (second element 20b) is effective in making the residual stresses thereof equal and therefore, the configuration of the reference element (second element 20b) shown in FIG. 6(b) is better than the configuration in which the first slit 35 and the second slit 36 are not provided in the reference element.
Incidentally, in the same manner as with other semiconductor devices, also in thermal-type infrared solid-state image sensing devices, technology development for downsizing has been pushed forward with. The downsizing of a thermal-type infrared solid-state image sensing device is achieved by the downsizing of the light receiving element and the like which constitute the thermal-type infrared solid-state image sensing device. In a light receiving element provided with a light receiving section (temperature detection section) and a supporting section in the same hierarchical layer, the area capable of being allotted to the supporting section decreases if the area of the light receiving section (temperature detection section) is increased, whereas contrastingly the area of the light receiving section (temperature detection section) is reduced if the area occupied by the supporting section is increased; this is unfavorable for downsizing. As in the above-described example of configuration shown in FIG. 6(a), due to the effect of the “eave,” it is possible to make the area of light receiving itself wide, making it possible to improve the aperture ratio. However, a decrease in the area of the light receiving section (temperature detection section) reduces the “bolometer thin film volume,” causing an increase in 1/f noise. That is, a decrease in the S/N ratio caused by an increase in 1/f noise occurs. Therefore, the configuration in which a relative decrease in the “bolometer thin film volume” can be avoided, for example, a light receiving element separately provided with a light receiving section (temperature detection section) and a supporting section is favorable for downsizing. The configuration of a thermal-type infrared solid-state image sensing device composed of a light receiving element provided with a light receiving section (temperature detection section) and a supporting section in separate hierarchical layers is disclosed in JP 2010-101756 A.
Referring to FIG. 7 and FIGS. 8(a) and 8(b), an explanation will be given of the configuration of the thermal-type infrared solid-state image sensing device disclosed in JP 2010-101756 A. FIG. 7 is a plan view (plan arrangement) showing the configuration of a light receiving element in this thermal-type infrared solid-state image sensing device disclosed in JP 2010-101756 A. FIGS. 8(a) and 8(b) are sectional views of the configuration (sectional configuration) of the light receiving element having the “plan arrangement” shown in FIG. 7. FIG. 8(a) schematically shows the configuration of an element (pixel) in the path from one supporting section to the other supporting section via a light receiving section (diaphragm). However, the division of the bolometer thin films and the metallic interconnect (third interconnect) which connects bolometer thin films are omitted. FIG. 8(b) is a diagram schematically showing the configuration of a plurality of elements (pixels) horizontally arranged side by side in FIG. 7 at the pitch of “light receiving section (diaphragm) length”+“gap between light receiving sections (diaphragms)”, and each element (pixel) shows the section obtained by being cut along the A-A′ line shown in FIG. 7.
As shown in FIG. 7 and FIGS. 8(a) and 8(b), the light receiving element in this thermal-type infrared solid-state image sensing device is made up of a light receiving section (diaphragm 38), a pair of supporting sections (a first supporting section 39 and a second supporting section 40) which support the light receiving section (diaphragm 38) in such a manner as to float from a Si substrate with a read-out circuit 45 (the read-out circuit is not shown). In the light receiving section (diaphragm 38), there is formed a bolometer thin film 52 divided into three parts as a temperature change detection mechanism and this bolometer thin film 52 is covered with a third insulating film (protective film) 51 on the lower layer side as well as a fourth insulating film (protective film) 53 and a fifth insulating film (protective film) 55 on the upper layer side. This bolometer thin film 52 is made of vanadium oxide (V2O3, VOx etc.), titanium oxide (TiOx) and the like having film thicknesses of the order of 30 nm to 200 nm. The third insulating film (protective film) 51, the fourth insulating film (protective film) 53, and the fifth insulating film (protective film) 55 are formed from a Si oxide film (SiO, SiO2), a Si nitride film (SiN, Si3N4) or a Si oxynitride film (SiON) and the like. The film thicknesses of the third insulating film (protective film) 51, the fourth insulating film (protective film) 53 and the fifth insulating film (protective film) 55 are on the order of 50 nm to 500 nm, 50 nm to 200 nm, and 50 nm to 500 nm, respectively.
The divided parts of the bolometer thin film 52 are connected in series by a third interconnect 54. The third interconnect 54 is made of aluminum, copper, gold, titanium, tungsten, molybdenum or alloys such as titanium-aluminum-vanadium and the like or semiconductors such as Si to which impurities are added in high concentrations, which have the film thicknesses of the order to 10 nm to 200 nm. The third interconnect 54 is covered with the third insulating film (protective film) 51 on the lower layer side as well as the fourth insulating film (protective film) 53 and the fifth insulating film (protective film) 55 on the upper layer side. Each of the third interconnects 54 provided in end portions of the bolometer thin film 52 which are connected in series, passes through the region narrowed by a slit 44 formed in a position adjacent to the first supporting section 39, and is drawn out up to a first contact section 42, whereby the third interconnect 54 along with the third insulating film (protective film) 51, the fourth insulating film (protective film) 53 and the fifth insulating film (protective film) 55 form the first supporting section 39.
In the above-described first contact section 42, a first interconnect 48 and a second interconnect 49 are formed on a first insulating film (protective film) 47 and connected to the third interconnect 54 by contact holes provided on a second insulating film (protective film) 50, the third insulating film (protective film) 51, and the fourth insulating film (protective film) 53. A first interconnect 48 and a second interconnect 49 are made of aluminum, copper, gold, titanium, tungsten, molybdenum or alloys such as titanium-aluminum-vanadium and the like or semiconductors such as Si to which impurities are added in high concentrations. The film thickness of the first interconnect 48 and the film thickness of the second interconnect 49 are on the order of 50 nm to 200 nm and on the order of 10 nm to 200 nm, respectively. The first interconnect (protective film) 47 and the second interconnect (protective film) 50 are both formed from a Si oxide film (SiO, SiO2), a Si nitride film (SiN, Si3N4) or a Si oxynitride film (SiON) and the like having film thicknesses of the order of 50 nm to 500 nm.
The second interconnect 49 is covered with the first insulating film (protective film) 47 on the lower layer side and the second insulating film (protective film) 50 on the upper layer side, passes a beam 41 which is bent in a complex manner, is drawn out up to a connecting electrode 46 provided on the Si substrate with a read-out circuit 45, and electrically connected to the connecting electrode 46 via the first interconnect 48 formed in a contact hole provided in the first insulating film (protective film) 47. The first insulating film (protective film) 47, the first interconnect 48, the second interconnect 49, and the second insulating film (protective film) 50 form the second supporting section 40 which is made up of the three parts of the first contact section 42, the beam 41, and a second contact section 43. Incidentally, the first interconnect 48 is provided in order to avoid problems, such as the piercing-through of the first contact section 42 during the formation of a contact hole and the step breaking of the second contact section 43 in a stepped part. The first interconnect 48 may not be provided when the film thickness of the second interconnect 49 is such a thickness as might exclude the danger of piercing-through and step breaking.
In the configuration of the thermal-type infrared solid-state image sensing device disclosed in JP 2010-101756 A, as described above, the light receiving section (diaphragm 38) and the beam 41 in the supporting section which determines the thermal separation performance are provided in different level. For this reason, when the area occupied by the beam 41 provided in the lower level is increased, the area occupied by the beam 41 has no effect on the area of the light receiving section (diaphragm 38) provided in the upper level and hence, this does not reduce the area of the light receiving section (diaphragm 38). That is, the configuration in which the light receiving section (diaphragm 38) and the beam 41 are provided in different level, which is disclosed in JP 2010-101756 A, is favorable for downsizing.